Project Abstract The proposed study aims to determine the effect of ECT on completed suicides in geriatric patients with major depressive disorder (MDD). Data will be drawn from both the Medicare datasets and the National Death Index. To examine the effect of ECT on suicide, we will use an instrumental variables analysis, which accounts for unmeasured confounders, applied to propensity score-matched data. Prior to applying the propensity score model, patients will be individually matched on age, gender, number of antidepressant trials and medically treated suicide attempts in the preceding year. The instrumental variable will be the proportion of patients with MDD treated with ECT in each hospital in the prior calendar year; this instrument has been used successfully in a previous study of ECT and 30-day readmission risk. The primary outcome of the comparison will be completed suicide; secondary outcomes will include all-cause mortality. Suicide is a major public health crisis and, despite renewed efforts by public health leaders, the suicide rate has been increasing in the last 15 years. Most suicide victims suffer from treatable psychiatric disorders, most commonly a mood disorder such as MDD. ECT is the most effective treatment for MDD and professional guidelines recommend ECT as a treatment for severely ill patients with mood disorders who are at high risk of suicide. However, there is a critical gap in our understanding of the relationship between ECT and suicide; there has not been a convincing and consistent link between and reduced risk of completed suicide. The proposed study aims to fill this gap. Our study focuses on an enriched sample of geriatric patients, who experience particular benefit from ECT and are more likely than younger patients to receive the treatment. Additionally, this age group is historically at high risk for suicide. The proposed study is of central relevance to the NIMH mission of reducing suicide rates. Despite ECT being the most effective and definitive therapy for severe depression, it has been dogged by stigma for decades. However, the last several years have seen a surge in high-quality ECT research published in prominent journals reiterating the effectiveness of ECT, developing algorithms for predicting clinical outcomes, improving longer-term outcomes, showing its potential benefits on population health outcomes, and suggesting it is most cost-effective when used after a 2nd failed antidepressant trial. Given this resurgence of ECT research, the proposed study is both timely and of critical importance to reducing suicide rates. If, as hypothesized, ECT is shown to be associated with reduced rates of ECT, future research could aim to increase the dissemination of this critical treatment and study barriers to treatment access. Furthermore, the lack of a consistent link between ECT and reduced suicide rates is often used by anti-psychiatry/anti-ECT advocates to protest the use of the treatment. If ECT can convincingly be shown to be associated with lower rates of suicide, this may serve to galvanize suicide prevention groups to demand greater access to the treatment.